A variety of semiconductor chip packages are known that provide support for an integrated circuit chip (IC) or die and associated bond wires, provide protection from the environment, and enable surface mounting of the die to and interconnection with a printed circuit board (PCB). One conventional package configuration is a flat no-leads package such as quad-flat no-lead (QFN) or and dual-flat no-lead (DFN). Flat no-lead packages, also known as micro leadframe (MLF) and SON (small-outline no leads), is a surface-mount technology, one of several package technologies that connect ICs to the surfaces of PCBs without through-holes. Perimeter lands or terminals exposed on the package bottom provide electrical connections to the PCB. Flat no-lead packages also include an exposed thermal die pad to improve heat transfer out of the IC (into the PCB).
QFNs and DFNs are relatively difficult to solder to because all the terminal connections are on the bottom of the package. Some designs have small extensions for these terminals that wrap around the bottom corner of the package and come up along the edge of the package somewhat. As QFN and DFN packages conventionally comprise copper leadframes, to allow soldering thereto they have added terminal finishes, such as comprising Matt Sn (100% tin) which is applied to the terminals using an electroless (or auto-catalytic by only chemical means) plating process. The QFN/DFN tin coated terminals can then be soldered to a PCB.
Inspection of QFN and DFN packages soldered onto a PCB is typically done with X-ray equipment for an assembly process monitor. The X-ray can detect solder bridging, shorts, opens, voids and solder joint fillets under the leads and exposed pads which are determined as the critical area of termination. For example, the J-STD-002C solderability standard includes a proposed (in ballot) soldering criteria for the QFN/DFN. Some customers (e.g., automotive customers) for QFN packaged semiconductor devices besides requiring a terminal underside solder inspection also require a side solder inspection.